This is their response :

Upon checking we see that there are Marketplace Withheld Tax / Marketplace Facilitator VAT-Principal charges on your orders which is based on the destination of your order, Amazon may be legally obligated to calculate, collect and remit applicable national, state or local sales or use taxes, goods and services taxes (GST), or value added taxes (VAT) (collectively referred to as ‘Taxes’).

MPF is commonly defined as a business that contracts with third-party sellers to “facilitate” the sale of physical property, digital goods, and/or services owned and supplied by the third party through an online store or “marketplace.” Based on this definition, Amazon is a marketplace facilitator.

Marketplace tax collection applies to all products, from all sellers, on all Amazon marketplaces, when an order is shipping to a jurisdiction (country, state, county, city, district) where Marketplace Facilitator laws have been enacted. All sellers must agree to automatic Marketplace Tax Collection, irrespective of their location, product or order origin, or the volume or quantity of sales made by the seller.

The buyer pays the Taxes collected by Amazon on the product sold. The collected amounts are reflected on your order details and order reporting with a deduction (using the reporting identifiers in the table below) equal to the amount of Amazon’s obligation to remit to the appropriate government authority.

The message "Net Proceeds will not include VAT for Marketplace Facilitator order". Marketplace Facilitator means that Amazon is liable for VAT. Where it states “Marketplace” then Amazon will remit the VAT to the authorities. Amazon will calculate and collect UK VAT from the customer at checkout and remit this directly to the UK Tax Authorities. You will not receive the UK VAT amount in your disbursements and will not be required to remit these amounts to the UK Tax Authorities.